Method for producing composite material

ABSTRACT

There is provided a method of manufacturing a composite material, which includes: manufacturing the composite material from a raw material including metal silicon and ceramic. The method of manufacturing the composite material is characterized in that the raw material includes metal silicon whose surface is coated with an organic matter. Metal silicon in which pinhole defects are not easily generated is included in the raw material in the method of manufacturing the composite material.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a compositematerial, which comprises manufacturing the composite material from rawmaterials including metal silicon and ceramic, particularly to a methodof manufacturing a composite material, in which pinhole defects are noteasily generated in the composite material.

BACKGROUND ART

A porous or dense metal-silicon-containing material containing metalsilicon, for example, a material in which a metal silicon-siliconcarbide composite material containing metal silicon and silicon carbideas a main phase is a main component, has a high strength, and issuperior in resistances to heat and oxidation, and thermal conductivity,and has therefore been used in various applications such as kiln tools,jigs for thermal treatments, honeycomb filters and the like at present.

This composite material can be manufactured in general, by kneading andpugging raw materials including metal silicon and ceramic such assilicon carbide, thereafter performing extrusion to form a formed bodyhaving a predetermined shape, and thereafter firing the body. Forexample, in a honeycomb structure for use in a diesel particulate filter(DPF) and the like, after forming a formed body having a honeycomb shapehaving a large number of cells partitioned by partition walls, the bodyis fired, and accordingly the structure can be manufactured.

However, coarse pinhole defects are sometimes generated in the compositematerial manufactured using metal silicon and ceramic as main rawmaterials. If the defects are generated, an original function of thecomposite material is sometimes impaired. For example, when the pinholedefects are generated in the partition walls of the honeycomb structure,the partition walls do not function. In the DPF using porous partitionwalls as the filter, soot to be trapped (i.e., soot-like matters) leaksthrough the pinhole defects on an exhaust side, and cannot be trapped,and a problem occurs that defective articles are made. Therefore, therehas been a demand for a method of manufacturing a composite material inwhich the pinhole defects are not easily generated.

DISCLOSURE OF THE INVENTION

The present invention has been developed in consideration of thesesituations, and provides a method of manufacturing a composite materialin which the above-described pinhole defects are not easily generated.

According to the present invention, there is provided a method ofmanufacturing a composite material, which comprises: manufacturing thecomposite material from a raw material including metal silicon andceramic, characterized in that the raw material includes metal siliconwhose surface is coated with an organic matter.

In the present invention, a content of metal silicon to a total contentof metal silicon and ceramic in the raw material is preferably 5% bymass or more, and the method preferably includes a step of forming theraw material into a predetermined shape by steps including a kneadingstep, a pugging step, and an extruding step. The method also preferablyincludes a firing step of firing the raw material, and furtherpreferably a firing temperature in the firing step is 1400 to 1600° C.The composite material is preferably a porous body having a porosity of30 to 90%, and a composite material having a honeycomb shape ispreferably manufactured. The ceramic included in the raw materialpreferably includes silicon carbide. The organic matter is preferably aresin, and the organic matter is also preferably a silicon-containingcompound.

BEST MODE FOR CARRYING OUT THE INVENTION

A method of manufacturing a composite material according to the presentinvention will be described hereinafter in detail, but the presentinvention is not limited to the following embodiment.

A raw material according to the present invention includes metal siliconand ceramic, and important characteristics of the present invention liein that the raw material includes metal silicon whose surface is coatedwith an organic matter.

As a result of detailed studies of causes for generation of pinholedefects in manufacturing the composite material using the raw materialincluding metal silicon, the present inventors have found that, ingeneral, during the manufacturing, water is added to the raw material,and kneading, pugging, and extruding are performed, and in this case, byuse of metal silicon as the raw material, metal silicon reacts withwater as in Formula (1) to generate hydrogen. Furthermore, it has beenfound that when hydrogen is generated in kneading, pugging, andextruding steps, a generated hydrogen gas gathers to sometimes formclosed pores in a formed body, and this is one of causes to finallygenerate coarse pinhole defects of about 500 to 1000 μm. Moreover, ithas been found out that metal silicon in the raw material is coated withthe organic matter to thereby suppress the reaction with water,accordingly hydrogen is inhibited from being generated, and the pinholedefects are not easily generated.Si+2H₂O→Si(OH)₂+H₂  Formula (1)

In the present invention, the content of metal silicon is not especiallylimited. When metal silicon is included in the raw material, thegeneration of hydrogen is suppressed by the present invention, and thegeneration of the pinhole defects is suppressed. However, when moremetal silicon is included in the raw material, an effect of the presentinvention is further clarified. Therefore, when using the raw materialincluding 5% by mass or more, further 10% by mass or more, especially15% by mass or more of metal silicon with respect to a total of 100% bymass of metal silicon and ceramic, a larger effect is preferablyobtained by the use of the manufacturing method of the presentinvention.

In the present invention, the shape of metal silicon for use as the rawmaterial is not especially limited, but, in general, shapes ofparticles, needles or the like are microscopically preferable, andpowder body shapes are macroscopically preferable. When metal siliconhas a shape of a powder body, the body is coated with the organicmatter, and this produces an effect that the powder body can beinhibited from being aggregated.

In the organic matter for use in the present invention, there is not anyspecial limitation to a type as long as metal silicon can be coated.However, from viewpoints that contact of metal silicon with water beinhibited and mutual aggregation of metal silicon be inhibited, such anorganic matter that the surface of metal silicon coated with the matterbecomes hydrophobic is preferable. In general, when manufacturing thecomposite material of ceramic, metal and the like, a calcining stepand/or a firing step for removing a binder and the like is performed inmany cases. Therefore, the organic matter is preferably removed in thecalcining step and/or the firing step, especially in the calcining step,for example, in the calcining step performed, for example, at 550° C. orless. Alternatively, preferably the organic matter does not adverselyaffect the produced composite material, even when a part or all of thematter remains in the composite material. For example, an element whichis burnt or oxidized and easily removed in the firing step, the sameelement as that included in the composite material or the like is apreferable main component, the element selected, for example, fromcarbon, hydrogen, oxygen, and silicon is preferably the main component,and silicon is especially preferably included. Concrete examples includealiphatic hydrogen carbide, fatty acid, fatty acid ester, silicone resinsuch as silicone oil, silicon-containing compound such as silane-basedcoupling agent, and various polymers, and the silicon-containingcompound is especially preferable.

An amount of the organic matter for use in the present invention is notespecially limited, the organic matter is usable in an amount with whichmetal silicon for use can be coated to such an extent that the effect ofthe present invention is produced, and the amount is changeable with anamount or a surface area of metal silicon. For example, 0.1 to 10 partsby mass, more preferably 0.2 to 5 parts by mass, further preferably 0.5to 3 parts by mass are preferably included with respect to 100 parts bymass of metal silicon. In the present invention, all of metal silicon inthe raw material is preferably coated with the organic matter, but allof metal silicon does not have to be necessarily coated, metal siliconis coated to such an extent that the effect of the present invention isproduced, and then coated metal silicon is usable together with uncoatedmetal silicon.

A method of coating the surface of metal silicon with the organic matteris not especially limited, but, for example, a slurry or a solution inwhich the organic matter for use in the coating is dispersed ordissolved beforehand is prepared, and the raw material is dispersed inthe slurry or the solution, and thereafter filtered and/or dried.Alternatively, a method in which the slurry or the solution is added atthe time of crushing of metal silicon or the like is considered.

The ceramic included in the raw material according to the presentinvention is not especially limited, as long as the ceramic is capableof forming the composite material with metal silicon. Concrete examplesof a preferable ceramic include silicon carbide, silicon nitride, andaluminum nitride. The examples of an oxide base include alumina,zirconia, yttria, mullite, cordierite, aluminum titanate and the like.

In the present invention, in addition to metal silicon and ceramic,various additives can be added to the raw material. For example, waxes,binders such as various polymers, dispersant, pore former, water and thelike may be appropriately added in accordance with purposes,applications, types of raw materials and the like.

Next, preferable manufacturing steps in the manufacturing method of thepresent invention will be described. In the present invention, first,for example, methyl cellulose and/or hydroxypropoxyl methyl cellulose isadded as a binder to a metal silicon powder coated with theabove-described organic matter, and a ceramic powder, further surfactantand water are added, and this raw material is mixed. This material iskneaded using a kneader in a kneading step so that a plastic clay can beobtained. The clay is pugged by a pug mill such as a vacuum pug mill,and thereby degassed in a pug milling step, and, for example, acylindrical preliminary formed body can be obtained. This body isextruded by an extruder in an extruding step, and accordingly a formedbody having a predetermined shape, for example, a honeycomb shape can beobtained. The formed body having the predetermined shape may be obtainedby continuously performing a series of kneading, pugging, and extrudingsteps by a continuous type extruder. The formed body obtained in thismanner is dried, thereafter calcined to remove the binder and the like,and fired in a firing step, and accordingly a composite material can bemanufactured. A firing temperature differs with the type of the ceramicfor use, but the body can be fired, for example, at 1400 to 1600° C.

When the composite material is manufactured by the steps including thekneading, pugging, and extruding steps as described above, and when thepugging step is performed with a reduced pressure or in vacuum,generated hydrogen easily generate closed pores in the raw material.Therefore, when manufacturing the composite material by the stepsincluding the kneading, pugging, and extruding steps, the manufacturingmethod of the present invention is especially effective. Since thehoneycomb structure has a large number of cells partitioned by partitionwalls and extending through an axial direction, the structure has alarge number of thin partition walls, functions of the walls are easilyimpaired by holed defects, and therefore the manufacturing method of thepresent invention is especially effective.

Moreover, when the composite material is manufactured by theabove-described series of steps, the composite material can be formedinto a porous body. When the composite material is a porous body,especially when the material is a porous body having a porosity of 30 to90%, the material is sometimes used in a filter and the like. Also inthis case, the pinhole defects raise a large problem, the function ofthe filter is largely impaired, and therefore the manufacturing methodof the present invention is effective. Especially, when the porous bodyis used as a DPF, the body is formed into the honeycomb structure inmany cases. Since the structure has a large number of porous partitionwalls, generation of coarse pinhole defects results in a problem thatsoot and the like to be trapped, discharged from a diesel engine, leakon an exhaust side. Therefore, the manufacturing method of the presentinvention for suppressing the pinhole defects is especially effective.It is to be noted that when the composite material is used in a filtersuch as a DPF, the formed body formed in the extruding step is formedinto a honeycomb shape, and further it is preferable to include aplugging step of alternately plugging adjacent cells in end surfaces onopposite sides. Furthermore, formed plugging portions are preferablyfired, and therefore the plugging step is preferably performed beforethe firing step.

EXAMPLES AND COMPARATIVE EXAMPLES

The present invention will further be described hereinafter in detailbased on examples, but the present invention is not limited to theseexamples.

Example 1

To 100 parts by mass in total of a raw material including: 80 parts bymass of a silicon carbide powder having an average diameter of 33.0 μm;and 20 parts by mass of a metal silicon powder having an averagediameter of 4 μm and including surfaces of particles, coated with asilicone resin, methyl cellulose and hydroxypropoxyl methyl cellulose,surfactant and water were added, and a plastic clay was prepared. Thisclay was degassed by a vacuum pug mill, preliminarily formed into acylindrical shape, extruded, and dried by microwave and hot air toobtain a formed body having a honeycomb structure having: a partitionwall thickness of 380 μm; a cell density (the number of cells per unitsectional area) of about 31.0 cells/cm² (200 cells/square inch); asquare section having one side of 35 mm; and a length of 152 mm. In theformed body, one end portion of a cell on an opposite side of anadjacent cell was plugged with a material similar to that used inmanufacturing the honeycomb structure so that end surfaces exhibitedcheckered patterns. After drying, the body was degreased at 400° C. toobtain a precursor. The obtained precursor was fired at 1450° C. toobtain the honeycomb structure for a DPF. With respect to the obtainedhoneycomb structure, holed defects in the honeycomb structure weredetected by a method described in Japanese Patent Publication No. 5-658.That is, by a method in which soot was passed via one end surface of thehoneycomb structure, and defects were inspected from an image ofparticles trapped by a screen attached to the other end surface, 1000honeycomb structures were inspected. As a result, there were 95% ofpassed articles including A ranked articles having zero detected holeddefect and B ranked articles from each of which three or less defectswere detected.

COMPARATIVE EXAMPLE

A honeycomb structure for a DPF was prepared in the same manner as inExample 1 except that metal silicon uncoated with a silicone resin, thatis, untreated metal silicon was used, and pinhole defects wereinspected. Detected A, B ranked passed articles were 85%.

Industrial Applicability

As described above, according to a manufacturing method of the presentinvention, pinhole defects are inhibited from being generated, and aquality level of a composite material can be enhanced. Therefore, themanufacturing method of the present invention is preferably usable inmanufacturing a composite material including metal silicon and ceramic,for use in various applications such as kiln tools, jigs for thermaltreatments, honeycomb filters and the like.

1. A method of manufacturing a composite material, which comprises:manufacturing the composite material from a raw material including metalsilicon and ceramic, characterized in that the raw material includesmetal silicon whose surface is coated with an organic matter.
 2. Themethod of manufacturing the composite material according to claim 1,wherein a content of metal silicon to a total content of metal siliconand ceramic in the raw material is 5% by mass or more.
 3. The method ofmanufacturing the composite material according to claim 1, comprising: astep of forming the raw material into a predetermined shape by stepsincluding a kneading step, a blunging step, and an extruding step
 4. Themethod of manufacturing the composite material according to claim 3,further comprising: a firing step of firing the raw material.
 5. Themethod of manufacturing the composite material according to claim 4,wherein a firing temperature in the firing step is 1400 to 1600° C. 6.The method of manufacturing the composite material according to claim 1,wherein the composite material is a porous body having a porosity of 30to 90%.
 7. The method of manufacturing the composite material accordingto claim 1, wherein a composite material having a honeycomb shape ismanufactured from a raw material.
 8. The method of manufacturing thecomposite material according to claim 1, wherein the ceramic included inthe raw material includes silicon carbide.
 9. The method ofmanufacturing the composite material according to claim 1, wherein theorganic matter is a resin.
 10. The method of manufacturing the compositematerial according to claim 1, wherein the organic matter is asilicon-containing compound.